This is not a bug – it´s a feature Leaflet Maps Marker Plugin – “Meine Platzl im Grätzl”-Edition has a built in function which checks the coordinates on new markers and layers. If the coordinates are within the boundaries of Vienna/Austria, the high-resolution maps from OGD Vienna (http://data.wien.gv.at) are preselected for you.
If you don´t want to use this feature (e.g. if you would always like to use OpenStreetMap for locations in Vienna), you can modify or disable this feature completely under Leaflet Maps Marker / Settings / Default Basemap / OGD Vienna Locator.
Posted in: Usage